So an aftermarket steering wheel would be a failure?
I asked my local MOT place the same question, they said alot of the MOT additions get revised as they are often unworkable.
They said as far as they're concerned if you have a steering wheel fitted and it's airbag is faulty it's a fail,
if they had a car in with an aftermarket steering wheel they'd more than likely pass it,
as in their opinion something can't be faulty if it's not there. (As long as the airbag light isn't on)
I'll see if thiey stick to that interpretaion come January when the new rules come into effect.